Process for the preparation of 2,3,5,6-tetrafluorobenzoic acid

Abstract

2,3,5,6-Tetrafluorobenzoic acid is prepared by hydrogenolysis of pentafluorobenzoic acid (esters).

What is claimed is: 
     
       1. A process for the preparation of 2,3,5,6-tetrafluorobenzoic acid comprising hydrogenating pentafluorobenzoic acid or an ester thereof in the presence of a hydrogenation catalyst, a base, and a solvent at a pressure of 2 to 100 bars, wherein 0.9 to 0.98 moles of hydrogen are employed per mole of pentafluorobenzoic acid wherein the catalyst is selected from the group consisting of palladium, cobalt, nickel, rhodium, ruthenium, iridium, rhenium and mixtures thereof, in metallic or oxidic form. 
     
     
       2. A process according to claim 1 in which the hydrogenation catalyst used is a palladium-containing catalyst. 
     
     
       3. A process according to claim 1 in which the base used is selected from the group consisting of sodium hydroxide and sodium carbonate. 
     
     
       4. A process according to claim 1 in which the solvent used is water. 
     
     
       5. A process according to claim 1 further comprising treating the crude 2,3,5,6-tetrafluorobenzoic acid obtained from the hydrogenation firstly with an alkali metal sulphide and subsequently with an oxidant to obtain purified 2,3,5,6-tetrafluorobenzoic acid. 
     
     
       6. A process according to claim 5 in which the reaction with the alkali metal sulphide and the oxidant is carried out in aqueous solution. 
     
     
       7. A process according to claim 5 in which the alkali metal sulphide is used in an amount which corresponds to the amount of unreacted starting compound. 
     
     
       8. A process according to claim 1, wherein the pressure is 5 to 60 bars. 
     
     
       9. A process according to claim 1, wherein the temperature is 20° C. to 120° C. 
     
     
       10. A process according to claim 1, wherein the process is conducted for 10 to 200 minutes. 
     
     
       11. A process according to claim 1, wherein the process is conducted for 30 to 120 minutes. 
     
     
       12. A process according to claim 1, wherein the solvent is an organic solvent.